I see the following is up for sale again. Is there no responsibility on the seller to produce some indication of the authenticity of an item like this? Or does one simply state that it is "highly speculative" and then pass off any rubbish as potentially valuable?

Or does one simply state that it is "highly speculative" and then pass off any rubbish as potentially valuable?

I think you have hit the nail on the head. Welcome to eBay, right? Just like a garage sale, flea market, boot-sale, etc. Buyer needs to beware at all times, and do some research. If a deal is too good to be true, it probably isn't true.